Admixtures are generally added to a concrete batch before or during mixing. A proper use of admixtures offers certain beneficial effects to concrete, mortars, and other masonry materials, including improved quality, acceleration or retardation of setting time, enhanced frost and sulfate resistance, control of strength development, improved workability, improved water repellency, and enhanced finishability.
Admixtures vary widely in chemical composition, and many perform more than one function. Two basic types of admixtures are available: chemical and mineral. Mineral admixtures (fly ash, silica fume, and slags) are usually added to concrete in larger amounts to enhance the workability of fresh concrete; to improve resistance of concrete to thermal cracking, alkali-aggregate expansion, and sulfate attack; and to enable a reduction in cement content. Chemical admixtures are added to concrete in small amounts mainly for the entrainment of air, reduction of water or cement content, plasticization of fresh concrete mixtures, or control of setting time.
Water-repellent additives have been incorporated into hydraulic cement-based products for decades, serving to resist moisture, such as rain water, from penetrating excessively into materials, such as masonry mortars. Reducing moisture penetration can help to resist mold and mildew, to protect against cracks, and to resist efflorescence stains. Reducing water absorption also prevents or minimizes the damaging effects of freeze-thaw cycles.
The degree of water repellency that is attained when a water-repellent admixture is added to a cement-based masonry mortar should comply with the performance set forth in ASTM C1384, Standard Specification for Admixtures for Masonry Mortars.
Water repellency should be achieved by the water-repellent admixture while avoiding a significant change in the air content of the cement-based material. By not significantly changing the air content, the workability of the plastic mortar as well as the properties of the hardened mortar including the compressive strength of the mortar and the bond strength of the mortar to other substrates, such as clay or concrete masonry units, are not adversely affected.
There remains a need to improve and optimize water-repellent admixtures for cementitious materials. Improved compositions are needed for water-repellent admixtures that may be incorporated during masonry processing and construction, to improve the water resistance of the final concrete, hardened mortar, grout, or other product. Additionally, dry water-repellent admixtures would be desirable so that the admixtures may be conveniently incorporated into pre-blended dry mortar mixes. It is known that liquid additives can pose difficulties during processing.